Different formats of optical recording medium including read-only optical discs, such as CD (Compact Disk), and DVD (Digital Versatile Disc); and recordable optical discs such as a CD-R (Compact Disc-Recordable), CD-RW (Compact Disc-Rewritable) and DVD+RW (Digital Versatile Disc+Rewritable); and Blu-ray discs (BD) are well known. These optical recording media may be written and/or read out by means of an optical pick up unit or a read head in an optical scanning device. The optical pick up unit is mounted on a linear bearing for radially scanning across the tracks of the optical disc. The read head may comprise, among other elements, an actuator for focusing, radial tracking and tilting the lens. The optical scanning device comprises a light source such as a laser, which emits light that is focused onto the information layer in the disc. In addition to detecting and reading the information from the optical disc, the optical pick up unit also detects a variety of error signals, e.g., focus error and radial tracking error. These error signals are used by the optical scanning device to adjust various aspects of the scanning procedure to help reduce these errors. For example, the focus error signal can be used to determine how much the focus actuator should be steered to improve the focus of the laser.
For optimal read back and recording on an optical disc system, the focus set point for the optical scanning device needs to be calibrated. However, on empty media, no HF quality measurement, e.g. jitter, bit error rate, byte error rate, etc, is yet available for producing a focus versus HF quality measurement curve. Only after the first optimum power calibration (OPC) procedure can some tracks be written in the OPC zone that can then be used for servo calibrations.
Unfortunately, especially with a push-pull tracking method, the servo margins are very narrow. Before a sufficient wide focus offset versus HF quality measurement curve can be obtained, the radial tracking servo can frequently fail. The track loss results in a failed focus offset curve 10, which is illustrated in FIG. 1. In FIG. 1, the track loss occurs after only 4 jitter measurements have been made. As a result, a reliable optimal focus offset for read and record cannot be found.
Because of the limited availability of space in the OPC area, only a few tracks are written. To obtain enough jitter measurements, one or more tracks need to be measured for each focus offset point. For each focus measurement point, the same track is measured again. Therefore, track jumping is needed before each jitter measurement can take place. Two known focus offset calibration methods are illustrated in FIGS. 2-3. In FIG. 2, the sub-optimum focus is set in step 202. The jitter measurement is then performed in step 204. The focus offset is then set in step 206. The system then seeks/jumps to a new track in step 208. After the track jump, the jitter is measured again in step 204. This process continues, in step 210, until jitter samples have been collected for all of the focus offset points on each of the available tracks, where the process ends in step 212.
The focus offset calibration routine illustrated in FIG. 3 is similar to the calibration routine illustrated in FIG. 2, however in this case the focus offset is set prior to the jitter measurements. As illustrated in FIG. 3, the sub-optimum focus is set in step 302. Then the focus offset is set in step 304. The jitter measurement is then performed in step 306. The system then seeks/jumps to a new track in step 308. After the track jump, the focus offset is set in step 304. This process continues to loop, in step 310, until jitter samples have been collected for all of the focus offset points on each of the available tracks, where the process ends in step 312.
In both known focus offset calibration routines, the seek/jump takes place with a focus setting that is different from the sub-optimal position. Because the seek/jump takes place with a non-optimal focus offset, some serious radial error distortion may occur, as illustrated in FIG. 7, which causes radial track loss. This results in large servo instabilities which result in undesired radial recoveries, track loss, and increased start-up time.
Thus, there is a need for a method and apparatus for an improved focus optimization routine.